Wireline logging measurements made in a wellbore are used to characterize rock formations. A typical run combines multiple tools connected together in series. The multiple tools often include one or more tools such as nuclear, acoustic, resistivity, and NMR tools. Typically, the measurement tools are reeled up the oil well at a constant speed with all of the tools continuously measuring the formation. The results of the multiple physical measurements made by the tools are often combined for interpretation.
In running multiple tools, the “slowest” tool determines measurement time. As the cost of rig time during logging is significant, logging time is an important factor when considering which measurements to collect. Thus, NMR as a “slow” measurement is generally limited from broader routine application in large part due to its slow logging speed.
In particular, NMR measurements typically characterize both the signal intensity (porosity) and T2 decay time (pore size and fluid types). Tool motion affects both of these quantities. The effect of tool motion on the signal intensity is due to long T1 recovery times. The T2 decay is also affected by motion because the detection region is of limited extent and typical tool motion over the longer relaxation times (˜1 s) can be comparable to detector size. Furthermore, there is significant additional signal decay due to the detector leaving the region from where the NMR signal originates. While some compensation is possible for this additional decay, existing NMR tools cannot achieve the desirable logging speeds of other tools (e.g., 1800 ft/hr) and still yield high quality NMR data.